Solar Thermal FAQs

Frequently asked questions about solar thermal systems

I have a Combi boiler. Is solar thermal an option for me?

These are some of the issues to consider if you have a Combi boiler:

• The hot water set-up will need to change to a system type with an unvented hot water cylinder.
• Space will need to be identified for the hot water tank. There will also need to be space for the expansion vessel, pressure reducing valve and solar thermal equipment. See approximate sizes below.

200L: 600mm diameter x 1500mm height

250L: 600mm diameter x 1750mm height

300L: 650mm diameter x 1750mm height

• A hot water cylinder is heavy so the floor structure will need to be adequate to support the weight of the tank.
• Building work may need to be accommodated to re-route pipework to the hot water cylinder location. This would include the space heating flow and return, hot, mains and balanced cold water pipework.
• Wiring would also be needed for the boiler thermostat cut off, back up immersion heater supply and solar thermal power supply.
• The combi boiler itself doesn’t necessarily need to be replaced. The cold pipework to the boiler can be isolated. New components would need to be fitted under the boiler including motorized valves, isolation valves, potentially magnetic particle filter etc.
• If you do not have space, or the building work would be too extensive, you may want to postpone a solar thermal system to another time.

I don’t want to have stored hot water, but want to keep my Combi system’s hot water on demand. Are there any other solar thermal options?

It is possible to get a dedicated solar pre-heat tank that would feed pre-warmed water to the Combi boiler.  A clever Combi diverter valve can be used in this setup. The boiler needs to have a minimum temperature difference to heat the water. If the water is hot enough the valve diverts the hot water to bypass the boiler. If the water isn’t hot enough, the valve mixes it with cold water to give the boiler a big enough temperature difference. If the temperature is above 48 degrees C, water goes straight to the tap. If water is between 48 and 28 degrees C, the solar pre-warmed water is mixed down to 28 degrees C. You still need a tank for this system with a dedicated electric element to sterilize the tank once a week. Flow rates would still be limited to the Combi boiler flow rate so there are limited benefits over a dedicated unvented hot water cylinder.

I have a system gas boiler and hot water tank, what would I need to be compatible with a solar hot water system?

A hot water cylinder with solar thermal heating and backup gas boiler heating needs to be the twin coil type. This means that both solar thermal system and gas boiler have a dedicated coil within the tank for indirect water heating.

• It is unlikely that an existing tank is twin coil, unless this was specifically requested. If the hot water tank is relatively new, replacement may not be preferable and installation of a solar thermal compatible tank may need to be postponed.
• If the existing hot water tank has been used for a long time it could be a good idea to replace this for a more efficient type which is also solar thermal compatible.
• Consider the new tank may be a larger capacity. There will also need to be space for the expansion vessel, pressure reducing valve and solar thermal equipment.

200L: 600mm diameter x 1500mm height

250L: 600mm diameter x 1750mm height

300L: 650mm diameter x 1750mm height

I have a direct electric hot water heating system. How do I install solar thermal water heating?

With a direct electric hot water heating system, a dedicated coil within the hot water tank would be needed for the solar thermal system. It is likely that you will therefore need to get a new hot water cylinder. It may be necessary to install a timer for the back-up immersion heater if not already present so that the electric backup can be programmed to come on at the end of the day.

Explanation of solar thermal components

Solar collector

A flat plate collector is specially designed to trap heat from the sun. A special black absorber surface collects solar irradiation and high quality glazing reduces heat loss from inside the collector. Even when it is 17 degrees outside the collector can reach 40-50 degrees.

Hot water cylinder

Heat from the solar collector warms the bottom of the cylinder next to the cold inlet. A back-up heating system e.g gas boiler increase the temperature of the cylinder to 60 degrees every evening when there is not sufficient heat from the solar thermal system alone

Controller

The Controller measures the temperatures in the collector and storage cylinder. When the collector temperature is hotter than the bottom of the hot water cylinder, the controller switches on the pump which circulates the heat from the collector into the tank.

Heat transfer fluid

An anti-freeze heat transfer fluid containing 40% mixture of propylene glycol and water is used to protect the external collector pipework down to -20 degrees C in the winter.

Pumpstation

The Resol Pumpstation contains a flow meter to measure the system flow rate, temperature dials to show how hot the heat transfer fluid is going to and from the collector, a high quality Wilo circulation pump, flushing and filling points to allow a flushing pump to purge air out of the system and the Resol controller.

Expansion vessel

As the heating fluid gets hotter it expands and an expansion vessel with an air membrane inside helps to keep the system pressure stable.

Discharge vessel

Sometimes when the system has not been adequately purged of air the system pressure can get very high and will escape out of the pressure relief valve. This fluid is captured in the discharge vessel so that the homeowner can see evidence of the problem and report this to their solar thermal engineer.

Kaspar Bradshaw, Project Engineer, Enhabit